It is well known that the texture, flavor, odor and appearance of many food products are significantly affected by the temperatures at which such products are prepared, stored and served. It has also been appreciated that moisture content, another important factor in the overall desirability or acceptability of food products, is greatly affected by the thermal treatment accorded to such products. Many complex chemical and physical changes in food products are brought about by the manner in which their temperature and moisture content are regulated.
Because of these and other considerations, numerous different apparatus and methods for heating and cooling food products have been disclosed in the prior art. Thus, for example, direct fired ovens, hot air convection ovens, infrared ovens and microwave ovens have all been utilized, either alone or in combination, to impart thermal energy to food products. Similarly, cooling has been accomplished by conventional refrigeration, flash-freezing, liquid freezant immersion, cryogenic freezing and the like. Particularly desirable and effective apparatus and methods for imparting thermal energy to food products have previously been disclosed in U.S. Pat. Nos. 3,884,213 and 4,154,861.
The apparatus and methods disclosed in the foregoing patents employ columnated jets of air or other gaseous fluid to create discrete areas of very high heat transfer on the surface of a food product. This high heat transfer accomplishes the advantageous effects of crisping, browning, searing, chilling, or freezing in those localized areas where the jets impinge against the surface of the product. When the jets are of proper size, spacing, temperature, velocity and direction, very desirable results are obtained. Furthermore, when the food products are passed beneath the jets at a selected rate, these effects are spread evenly over the surface.
Calculations, measurements of heat transfer and evident results on products heated illustrate that the impingement of jets of air substantially perpendicularly to the surface of the product accomplishes heat transfer rates of 2 to 10 times greater than heat transfer of the same volume of air moving parallel to the product surface. This relatively new, very rapid heat transfer is desirable to initiate rapid cooking, baking, cooling or freezing. However, because most food products and recipes or procedures for their preparation are designed with the thought in mind of heating or cooling those products by conventional means, they may not be prepared satisfactorily if they are only subjected to intensive impingement heating or cooling as previously disclosed. Although impingement heating or cooling can very quickly bring the surface temperature of the food products to very near that of the impinging gas, interior portions of those products may not be satisfactorily prepared by intensive impingement heating or cooling alone. Apparatus and methods are therefore needed for thermally treating food products in such manner that both the surface and interior portions of those products are satisfactorily prepared.
One method previously disclosed for heating both the surface and interior portions of food products is through use of microwave radiation in combination with impingement heating. When microwave energy is employed in combination with the columnated jets of heated gas impinging on the surface of the food products, internal and external heating can be accomplished simultaneously. However, the use of microwave energy is not well suited for all situations. For example, when using microwave energy, particular care must be taken to seal the enclosure in which the cooking is done in order to prevent the escape of potentially dangerous and harmful microwave radiation. Such difficulties become even more significant when dealing with conveyorized industrial or institutional sized ovens. Also, while microwave energy is effective for heating the interior portions of food products, there is no analogous apparatus or method that is similarly effective for cooling the interior portions of food products.
Therefore, an apparatus and method are needed for rapidly and efficiently heating and/or cooling both the surface and interior portions of food products. An apparatus and method are also needed that can further reduce the time and energy required to sequentially heat and cool food products, as is required in the preparation of many precooked, chilled or frozen foods.